This invention relates, generally, to an actuation system for an aircraft and, more specifically, a harmonic drive assembly of a rotary actuator with selective disconnect and reconnect.
Harmonic drive can reduce backlash in a motion-control system. More specifically, a harmonic gear allows high reduction ratios with concentric shafts and relatively low backlash and vibration. The harmonic gear is based upon a relatively simple construction using elasto-mechanical properties of metal.
The harmonic gear is constructed generally of a wave generator, flex spline, and ring gear. The wave generator is an oval or elliptical cam with a thin ball bearing placed around an outer circumference of the cam. The wave generator is mounted onto a shaft of a motor. The flex spline is thin and made of elastic metal with external teeth formed along an outer circumference of the flex spline. The ring gear is a rigid internal gear with internal teeth formed along an inner circumference of the ring gear. The internal teeth are of a same size as the external teeth of the flex spline, but the ring gear has more internal teeth than the flex spline has external teeth. The ring gear is attached to a gearbox along an outer circumference of the ring gear. The wave generator, flex spline, and ring gear are co-axially aligned with respect to each other.
In operation, the wave generator is mounted on a drive shaft, and, as the wave generator is rotated, it imparts a continuously moving elliptical form or wave-like motion to the flex spline. This causes meshing of the external teeth of the flex spline with the internal teeth of the ring gear at respective equidistant points of engagement or connection thereof to progress in a continuously rolling fashion. It also allows for full-teeth axial disengagement or disconnection at the points opposite a minor axis of the wave generator. A major axis of the flex spline actually rotates with the wave generator, so the points where the corresponding teeth mesh with each other revolve around a center point at a rate equal to that of the wave generator.
Since the flex spline has fewer external teeth than the ring gear has internal teeth and the full-teeth axial disconnect is made possible by the ellipticity of the wave generator, each complete revolution of the wave generator causes a circumferential displacement of the flex spline in relation to the ring gear. This displacement is always in a direction opposite to that of the rotation of the wave generator (i.e., if the wave generator is rotating in a clockwise direction, the displacement of the flex spline is in a counterclockwise direction and vice versa). In this way, the harmonic drive is capable of functioning as a speed reducer.
In an aircraft system, for example, a rotary electromechanical actuator (rotary EMA) is implemented mainly for so-called “secondary flight controls” (such as ailerons, flaps, slats, and spoiler and other flight-control surfaces). In this context, the harmonic drive is incorporated with the rotary EMA. When the rotary EMA fails, it can lock up a surface that it is configured to actuate. Upon such failure, it is desired to freely or selectively disconnect the rotary EMA and then reconnect the disconnected rotary EMA. However, axial movement of the wave generator out of the flex spline can render such reconnect difficult. As such, it is desirable to more easily reconnect the disconnected rotary EMA.